In recent years, contactless power transmission systems that supply power wirelessly, that is, in a non-contact manner have been actively developed. A method that attracts attention to realize contactless power transmission is the magnetic resonance method. The magnetic resonance method uses electromagnetic coupling between a transmitting coil and receiving coil for power transmission. By actively using a resonance phenomenon, the magnetic resonance method is characterized in that the amount of magnetic fluxes shared between a power supply source and a power supply destination may be small.
According to the widely known electromagnetic induction method, the degree of coupling between a transmitting side and a receiving side is very high and power can be supplied with high efficiency. However, because it is necessary to maintain the coupling coefficient at a high level, power transmission efficiency between coils on the transmitting side and the receiving side (hereinafter, called an “inter-coil efficiency”) is greatly degraded if the transmitting side and the receiving side are widely apart or displaced. On the other hand, the magnetic resonance method is characterized in that the inter-coil efficiency is not degraded when a Q value is large even if the coupling coefficient is small. That is, the need to adjust the axes of the transmitting side coil and the receiving side coil is advantageously eliminated and also a high degree of flexibility in positions of the transmitting side and the receiving side and the distance therebetween is provided. The Q value is an index of a circuit having the transmitting side or receiving side coil to represent the relationship between conservation and losses of energy (indicating the strength of resonance of a resonance circuit). The inter-coil efficiency will be described again later.
One of the most important elements in a contactless power transmission system is measures against heat of metal foreign matter. When power is supplied in a non-contact manner regardless of the electromagnetic induction method or the magnetic resonance method, an eddy current is generated if a metal is present between the transmitting side and the receiving side and the metal may be heated. To reduce the heat generation, many techniques to detect metal foreign matter have been proposed. For example, a technique using an optical sensor or a temperature sensor is known. However, a detection method using a sensor will be expensive when the range of power supply is wide like when the magnetic resonance method is used. In addition, when a temperature sensor is used, an output result of the temperature sensor depends on thermal conductivity therearound, which imposes design restrictions on devices on the transmitting side and the receiving side.
Thus, a technique to judge whether metal foreign matter is present by checking changes of parameters (the current, voltage and the like) when metal foreign matter comes between the transmitting side and the receiving side is proposed. Such a technique can reduce the cost without the need to impose design restrictions or the like. For example, Patent Literature 1 proposes a method of detecting metal foreign matter based on the degree of modulation during communication between the transmitting side and the receiving side and Patent Literature 2 proposes a method of detecting metal foreign matter based on eddy-current losses (foreign matter detection by DC-DC efficiency).